The proposed project has two main objectives: (1) determine the structure of a membrane protein complex and (2) relate structural changes to specific functional states. The protein complex which will be studied is the light-harvesting chlorophyll a/b complex (LHC-II) found in photosynthetic membranes of green plants. LHC-II is involved in maintaining the complex structure of the membranes and in regulating the relative amounts of energy delivered to photosystems I and II. LHC-II itself is responsive to cations and is regulated by phosphorylation and dephosphorylation, a control mechanism common to many membrane proteins. The specific aims of the project are: determination of the three- dimensional structure by the use of image analysis on cryo- electron images of two-dimensional crystals; labeling of the same crystals with monoclonal antibodies to localize specific polypeptides, followed by image analysis; modification of the crystalline complexes by phosphorylation and other procedures, followed by image analysis, and obtaining the high resolution structure of LHC-II by x-ray analysis of three-dimensional crystals. Work with two-dimensional crystals will determine the overall structure of the complex, the location of individual polypeptides and give an indication of structural changes which occur with changes in functional states. Work with three- dimensional crystals will localize individual amino acids. By combining information obtained from both techniques, it will be possible to gain insight as to how this complex functions. This information and further development of the methodologies used will help in understanding membrane proteins in general, a class of protein whose structural-functional relationships remain almost entirely unknown